Question

In which mode of expression, the concentration of solution remains independent of temperature?
A) Normality
B) Formality
C) Molality
D) Molarity

Answer 1

Hint: Compare the mode of expression which are given in options with respect to the relation to the volume of the solution. The terms which are dependent on the volume also affect the concentration of solution based on the change in temperature.

Complete step by step answer:
1) Let's analyze each chemical term given in the options one by one and decide which mode of expression is dependent on the temperature.
2) The term Normality is defined as the number of grams of mole equivalents of solute which is present in one liter of a solution. As the normality mode of expression is dependent on the volume of one liter of solution it is also dependent on the temperature. As the temperature increases the concentration decreases. Hence, this option is incorrect.
3) The term Formality is defined as the number of grams of molecular formula dissolved per liter of solution. As the formality mode of expression is dependent on the volume in a litre of solution it is also dependent on the temperature. As the temperature increases the concentration decreases. Hence, this option is incorrect.
4) The term Molality is defined as the number of moles present per kilogram of the solution. If there is an increase in the temperature of a solution then there will be no change in its mass as only the liquid gets evaporated by the temperature and also there will be no change in its number of moles. Hence, depending on the temperature there will be no change in the molality of the solution. Hence, this option is correct.
5) The term Molarity is defined as the number of moles present per litre of solution. As the molarity mode of expression is dependent on the volume in a litre of solution it is also dependent on the temperature. As the temperature increases the concentration decreases. Hence, this option is incorrect.

Therefore, option C is the correct choice.


Note:
The mode of expressions normality, molarity, and formality are related to the volume term of the solution. The volume of the solution changes as there is a change in temperature because the liquid part gets evaporated. Thus these terms do not remain constant with the change in temperature.